Primary keys in Cassandra are unique. Therefore there is no difference between an insert and an update in Cassandra; they are both treated as a write operation. Given that its underlying data files are immutable, it is possible that multiple writes for the same key will store different data in multiple files. The overwritten data doesn't automatically go away. It becomes obsolete (due to its timestamp).
When Cassandra processes a read request, it checks for the requested data from both memory and disk. If the requested data was written recently enough, it may still be in memory. Additionally, if data was written multiple times for a key that has been persisted to disk, it could be spread across multiple SSTable files. ...
